Ink jet recording method and apparatus

Abstract: A plurality of recording heads capable of ejecting inks of different colors is reciprocated relative to a recording material to print ink dots on the recording material while effecting main scanning in the forward and the backward pass thereof. A plurality of thinning patterns produced by preparing m x n unit picture element groups and arraying these unit groups in such a manner that the groups avoid mutually adjoining and assume a mutually complementary relation is used to effect a plurality of rounds of main scanning in the forward and the backward pass over an area which can be printed by one round of main scanning, record a thinned image, and complete the recording of an image. The array of picture element groups in the thinning patterns is determined so that the groups may mutually adjoin relative to the direction of main scanning.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an ink jet recording method and apparatus for recording an image on a recording material by ejecting ink droplets in conformity to the data of the image. Related Background Art In consequence of the dissemination of reproducing devices, information processors such as word processors and computers, and communication devices as well, digital image recording devices adapted to operate by use of an ink jet type recording head have been finding widespread acceptance among other image recording devices serving the apparatuses mentioned above. As a recording head having a plurality of recording elements arrayed integrally therein (hereinafter referred to as a "multi-head"), the recording devices of this kind are generally provided for the sake of enhancing their recording speed with a modified multi-head which has a plurality of ink nozzles and conduits integrally arrayed therein. Further, to permit production of color images, they are provided with a plurality of such multi-heads. Unlike the monochromic printer which prints characters exclusively, the color printer needs to fulfill various factors such as color development property, gradient of tone, and uniformity in printing color images. As regards the uniformity in particular, even slight inconstancy possibly caused among nozzle units by a deviation involved in the process of manufacture of a multi-head affects the amounts of ink droplets discharged through individual nozzles and the directions in which the ink droplets are ejected in the course of printing and eventually impairs uniform density of a printed image and deteriorates the quality of the produced image. A concrete example of this adverse case will be depicted below with reference to Fig. 11A to Fig. 11C. In Fig. 11A, 91 stands for a multi-head which is identical with a multi-head shown in Fig. 12A. Here, for the sake of simplicity, this multi-head is assumed to comprise 8 multinozzles 92. Denoted by 93 are ink droplets which are ejected by the multinozzles 92. Generally, the ink is ideally ejected in droplets of a uniform amount in parallel directions as illustrated in the diagram. If the ink is ejected as just described, then ink dots of a uniform size will land on the paper surface as illustrated in Fig. 11B and form a wholly uniform image free from uneven density (Fig. 11C). Actually, as pointed out previously, the individual nozzles betray inconstancy of quality among themselves. If they are used for printing in spite of the defect, the ink droplets ejected therethrough will vary in size and direction as illustrated in Fig. 12A and land on the paper surface as illustrated in Fig. 12B. From this diagram, it is clearly noted that blank areas unfulfilling the area factor of 100% occur periodically relative to the main scanning direction of the head, dots overlap one another to an unduly large extent on the contrary, or white streaks occur as found in the central part of the diagram. The aggregate of the dots which have landed in this condition describes a density distribution shown in Fig. 12C relative to the direction in which the nozzles are arrayed. As a result, these phenomena are generally perceived as uneven density by the human eyes. The following method has been proposed as a measure to this uneven density. The method will be described below with reference to Fig. 13A to Fig. 14C. This method requires the multi-head 91 to make three passes (or three scans) to complete such a print area as shown in Fig. 11A to Fig. 11C. One half of the area consisting of four picture element units is completed by two passes of the multi-head 91. In this case, the eight nozzles of the multi-head are divided into two groups, namely the upper and the lower group of four nozzles. The dots printed by one nozzle in one pass (or one scan) are such as result from thinning relevant image data roughly to one half in accordance with a prescribed image data array. Then, in the second scan dots corresponding to the remaining half image data eye filled or recorded, whereby the printing of the area of four picture element units is completed. The recording method just described will be hereinafter referred to as the "split recording method." This split recording method halves the effects inherently exerted by the individual nozzles on a printed image even when the recording head used therein happens to be equal to the recording head shown in Fig. 12A. The printed image consequently obtained is as shown in Fig. 13B. In this printed image, such black streaks or white streaks as are found in Fig. 12B are not very conspicuous. The uneven density of image, as shown in Fig. 13C, is appreciably alleviated as compared with what is shown in Fig. 12C. In the recording of image thus performed, the first and the second scans split the image data in a mutually offsetting manner in accordance with a prescribed array. Heretofore, as the image data array (thinning pattern), it has been most popular to adopt such an image data array as to produce a pattern resembling a shepherd's check by each set of one longitudinal and one lateral picture element. In the unit printing area (composed of four picture element units), therefore, the printing is completed by the first scan which prints a shepherd's check and the second scan which prints an inverted shepherd's check. Figs. 14A, 14B, and 14C portray how the record in a given area is completed with these shepherd's checks and inverted shepherd's checks by use of a multi-head provided with eight nozzles as illustrated in Figs. 13A to 13C. First, the first scan records a shepherd's check • by use of the four lower nozzles (Fig. 14A). Then, the second scan records an inverted shepherd's check ○ by feeding the paper across four picture elements (one half of the head length) (Fig. 14B). Further, the third scan again records a shepherd's check by feeding the paper across four picture elements (one half of the head length) (Fig. 14C). The record area of four picture element units is completed for each scan by sequentially alternating the feeding of the paper across four picture element units and the recording of a shepherd's check and an inverted shepherd's check as described above. Since the print is completed in one and the same area by use of two different kinds of nozzles as described above, this method permits production of an image of high quality free from uneven density. The drawing depicts the split recording method as adapted to complete a record in one and the same area by two passes. It should be remarked, however, that the effect of the split recording method gains in conspicuity in proportion as the number of groups into which the nozzles are divided increases. Even the recording apparatus described above is enabled to complete an image in one scanning direction by use of four kinds of nozzles when the number of picture elements to be recorded by one pass is further halved and'the scan for paper feeding is given a scanning width of two picture elements (one quarter of the head length). Thus, this apparatus is capable of producing a smoother and more desirable image. The split recording of this principle, however, has the disadvantage that the time cost for printing a given image on one paper surface increases and the throughput consequently decreases inevitably in proportion as the number of groups into which the nozzles are divided increases. For the purpose of further expediting the printing operation in this case, a method of adapting a carriage to produce printing and scanning operations on both the forward and backward directions may be conceived. In fact, this method can substantially halve the recording time spent normally heretofore on one paper surface because it wholly eliminates the motion which the carriage would have otherwise produced in returning idly to its home position after the recording by one pass has been completed. Actually, the monochromic printing apparatuses which adopt the principle of reciprocating printing are not few. Figs. 15A to 15D illustrate a head in the process of moving at a fixed velocity V in the forward or the backward direction-while causing an ink drop to be ejected at a fixed velocity v to a smooth paper surface. When the paper surface is flat and smooth as shown in Fig. 15A and, therefore, the distance d between the paper surface and the head face is always constant, the timing of the reciprocating printing motions of the head with respect to the discharge of ink is so set in advance that the position of the dot printed on the forward pass may coincide with that of the dot printed on the backward pass. When the paper surface is somehow caused to rise from the normal level as shown in Fig. 15C and, consequently, the distance d between the head face and the paper surface becomes shorter (d') and the interval between the time the ink drop departs from the head and the time it reaches the paper surface differs from the actually preset interval in each of the forward and the backward directions, the positions of the printed dots deviate from the target position as shown in Fig. 15D. Now, the detriment which arises when a thinning mask is used in the condition described above and the production of a 100% duty image is attempted by the reciprocating printing operation will be described below. Figs. 2A and 2B illustrate examples of the picture element array obtained by a varying recording scan in accordance with the split recording method designed to complete an image by four recording scans and the condition of a print of dots obtained therein. The picture element array to be printed by each recording scan has thinning masks (a) to (d) for the picture element arrays which are complementary to one another. In this case, the picture element array of the thinning mask used for the first recording scan and that used for the third recording scan are printed during the forward pass of the head, while those used for the second and the fourth recording scan are printed during the backward pass of the head. In the diagram, the number of picture elements recorded in the forward pass and that recorded in the backward pass are equal. If the deviation of dots shown in Figs. 15C and 15D occurs under this condition, the dots printed during the forward pass and those printed during the backward pass will deviate from each other and give rise to gaps between the horizontal rows of dots and between the vertical columns of dots to the extent of imparting a coarse appearance conspicuously to the produced image as shown in Fig. 2B. The image produced in this state betrays uneven density and inferior linearity of characters and lines because of the uneven arrangement of dots. As described thus far, an attempt to realize the split recording and the reciprocating printing simultaneously for the purpose of exalting the quality of image and expediting the printing operation results in yielding a printed image impaired by the deviation of dot positions which occurs during the reciprocating printing described above. The split recording method of this principle has been already disclosed as in JP-A-60-107,975 and USP 4,967,203. It is described as profoundly effective in countering such adverse phenomena as uneven density and lengthy streaks. The former patent specification defines this method as "being characterized by comprising means to assign a smaller width to the area of paper feed by each main scan than the width of said main scan and impart an overlapping part to the widths of two adjacent main scans and means to array printed dots in said overlapping part in such a manner as to prevent said printed dots from overlapping each other during said two main scans". According to the method of this specification, the thinning masks are so adapted as to "print an odd-number stage and an even-number stage alternately every other row", or to print an odd-number stage by the first main scan and an even-number stage by the second main scan, or alternatively to produce random recording by each pass. The thinning masks and the paper feed widths are not completely defined. In contrast, the latter US patent specification (USP 4,967,203) discloses this method as "comprising the steps of a) printing by the first pass the non-adjoining but alternating pixel positions in the horizontal and vertical directions only in the upper half part of the first band, b) printing by the second pass the pixels which have escaped being printed by said first pass in said first band and the non-adjoining but alternating pixels in the horizontal and vertical directions in the lower half part of said first band, and c) printing by the third pass the pixels in said first band which have escaped being printed by said first and second passes and, at the same time, effecting a first pass in the immediately following band". Thus, the invention of this US patent specification defines the thinning masks for effecting the split recording as non-adjoining but alternating pixels arranged in the vertical and horizontal directions. As a supplemental element, the invention under discussion discloses a recording method which comprises forming a pseudo pixel (superpixel) with an aggregate of several picture elements for the sake of gradient expression or multicolor expression and producing non-adjoining but alternating thinned prints with the superpixel units in the horizontal and vertical directions. Regarding this method, the specification has this passage: "Once the system for embodying this method is incorporated in a program software or a printer formware, the program of the system can be retrieved with a combined color number designated with respect to relevant superpixels and, therefore, the quality of print in question can be accomplished without indiscriminately complicating the work of forming a computer program for the production of a host of colors." The simplification of programming for the multicolor expression is adduced as one of the effects of this invention. Further, a mention is made to the effect that since the individual superpixels are intended to be perceived as unique uniform colors, the color bleeding possibly occurring in the superpixels is harmless. Incidentally, the split recording described above is at a disadvantage in requiring a large time cost for printing on one paper surface and entailing an inevitable decrease in the throughput. For the purpose of further curtailing the time spent in printing in this case, a method of adapting the carrier to produce reciprocating printing and scanning may be conceived. This method, in fact, can substantially halve the time required for recording on one paper surface because it wholly eliminates the motion which the carriage would have made otherwise in idly returning to its home position after the recording by one pass has been completed. Actually, the monochromic printing apparatuses which adopt the principle of reciprocating printing are not few. A color ink jet apparatus constructed as contemplated by the present invention had not yet been realized for the following reason. Figs. 17A to 17D are cross sections illustrating drops of the recording inks of popular use today in the process of falling onto a paper surface and subsequently diffusing in the wall of the paper. The diagrams represent the case of causing drops (dots) of the two inks different in color to fall with a time lag at two virtually adjoining positions on the paper surface and then diffuse (recording) in the paper. What should be remarked in this case is the fact that, in the part of the paper at which the two dots have overlapped each other, the dot which has landed the paper later tends to sink farther in the direction of thickness of the paper than the dot which has landed the paper earlier. This phenomenon may be logically explained by postulating that during the physical and chemical union of the coloring matter like a dye in the ejected ink with the recording medium, since the union of the recording medium with the coloring matter has its own limit, the union of the coloring matter of the earlier ejected ink with the recording medium proceeds preferentially and, therefore, the dot of this ink remains much on the surface of the recording paper and the coloring matter of the ink landing the paper later is not easily bound on the surface of the recording medium but left sinking farther in the direction of thickness of the paper and lodged fast in the depth of the paper unless the strength of union widely varies with the kind of coloring matter. If the two inks different in kind are printed at one and the same position in this case, the preference of coloring is varied by the sequence in which the two inks land on the paper. As a result, they will have expressed two different colors to the visual characteristics of man. It is now assumed that the colors of a four-color head are sequentially arranged from right in the order of black, cyan, magenta, and yellow and the head is reciprocated in the direction of arrangement of these colors (left to right) to effect a main scan. In the forward pass (or forward scan), the head is moved to the right and simultaneously caused to perform a recording action. At this time, the order of recording colors conforms to the aforementioned order in which the colors are arranged. When a green (cyan + yellow) signal is injected in a certain area, for example, cyan and yellow inks are absorbed in each of the affected picture elements in the order mentioned. In the present scan, therefore, the earlier absorbed cyan forms a preferential color by the mechanism of union described above and imparts a chromatic taste strongly of cyan to the eventually produced green dot. Conversely, in the backward pass (or backward scan) which follows the paper feeding made in the direction of a secondary scan, the head is moved in the direction opposite to that of the forward pass and simultaneously caused to effect a recording action. As a result, the order of injection of the color inks is reversed. In the present scan, a chromatic taste strongly of yellow is imparted to the produced green dot. When the scanning operation described above is repeated, green dots having a chromatic taste strongly of cyan and green dots having a chromatic taste strongly of yellow are recorded in accordance with forward passes and backward passes which are made by each of the recording heads. If the paper feeding is made by the width of the head as a unit for each of the forward and backward passes without using the split recording method in each pass, the areas of green having a chromatic taste strongly of cyan and those of green having a chromatic taste strongly of yellow will be alternated repeatedly by the width of the head as a unit and, as the result, the eventually produced green image which ought to be uniform in color will raise to serious degradation of quality. This drawback, however, can be overcome more or less by adopting the conventional split recording method demonstrated hereinabove. Specifically, in the operation of split printing, green dots having a chromatic taste strongly of cyan are recorded by the forward pass (Figs. 14A and 14C) and green dots having a chromatic taste strongly of yellow are recorded by the backward pass (Figs. 14B) as already explained with reference to Figs. 14A to 14C. The overall chromatic taste in a stated area, therefore, is moderated by the intermixture of the dots of the two chromatic tastes. The construction and effect of moderating the uneven coloration within the individual bands by intermixing the dots of two chromatic tastes within the stated area as described above have been already disclosed in USP 4,748,453. Though the US patent specification does not specify any limit for the amount of paper feeding, it has a mention to the effect that the method of this invention has the effect of preventing the beading of ink on the medium such as of an OHP grade paper by causing the first and the second (or more) split recording scan to perform a complementary recording on the picture elements alternately situated in the horizontal and vertical direction in one and the same area and, at the same time, preventing the color banding (uneven coloration) during the formation of a color image by reversing the order of ink injection into picture elements of a mixed color in the first pass (or first scan) and the second pass (or second scan) (reciprocating recording). Since the invention under discussion primarily aims to prevent the beading between the adjacent picture elements, it is characterized by the fact the picture elements which are recorded by one pass (or one scan) are alternated (not mutually adjoined) in the horizontal and the vertical direction. JP-A-58-194,541 which has issued to the same applicant as the present invention discloses a technique for the operation of an apparatus which is provided with a plurality of parallelly arranged series of recording elements and adapted to effect the main scan of a record of a dot matrix by reciprocating the head in a direction perpendicularly intersecting the series of recording elements mentioned above. This technique, in the operation mentioned above, comprises causing a smaller number of dots than the total number of dots destined to be recorded in at least either of the columns and rows of the dot matrix to be recorded intermittently in the forward pass of the main scan and, at the same time, causing the remaining dots in at least either of the rows and columns of the matrix to be intermittently recorded in the backward pass of the main scan thereby varying the order of overlapping of record in the overlapped recorded dots produced by the aforementioned plurality of series of recording elements in the forward and the backward pass of the main scan. The invention under discussion, unlike the split recording already described, has no restriction designed to decrease the number of rounds of paper feeding from the ordinary number and, as a result, succeeds in preventing a recorded image from being degraded in quality by the deviation of color tone (uneven coloration) due to the overlapped recording of color inks. Since this invention primarily aims to prevent deviation of color tone, it specifies no specially limited positions for the dots to be recorded by each pass. In the working examples of the invention cited in the patent specification, the lateral thinning for effecting alternate recording only in the longitudinal direction and the longitudinal thinning for effecting alternate recording only in the lateral direction are mentioned in addition to the recording in a checkerwise pattern (a shepherd's check and an inverted shepherd's check). Besides, JP-A-55-113,573 discloses a construction for effecting a reciprocating recording by use of a twill line (a shepherd's check and an inverted shepherd's check) pattern, though not limited to a color printer. The invention in this case aims to prevent the phenomenon of distortion of dots by avoiding continuous printing of adjacent dots and allowing an immediately succeeding dot to be printed before the immediately preceding dot dried up. Like the invention of USP 4,748,453 mentioned above, therefore, the invention under discussion limits the thinning masks to the twill line pattern. Incidentally, the inventions of the three patents cited above invariably aim to prevent the uneven coloration or beading in the course of reciprocating recording. They, therefore, avoid adopting the construction for "decreasing the amount of paper feeding between adjacent passes to below the ordinary width of heat" for the purpose of preventing the uneven density due to the inconstancy of nozzles in quality unlike the split recording method demonstrated hereinabove. It has been held that the reciprocating multicolor recording is feasible in spite of being susceptible of uneven coloration because the adoption of the split recording method for the reciprocating recording enables two kinds of recording picture elements for which the order of injection of ink colors is mutually reversed to be uniformly imparted in the recording area. The drawback of uneven coloration is not thoroughly overcome even when the split recording is effected in the pattern of the shepherd's check/inverted shepherd's check mentioned above. The reason for this failure follows. Generally, the amount of an ink droplet is so designed that it may spread on the paper surface to an area larger than the area assigned to a picture element. The larger area is necessary for wholly concealing the part of white paper surface in the area of data of printing ratio of 100% Even when the split recording method is followed, therefore, the printing medium (recording paper) has practically 100% of the area thereof covered notwithstanding only 50% of the recording picture elements are printed as illustrated in Fig. 16. A cross section taken through this recording paper is illustrated in Fig. 18A to Fig. 18C. The diagram represents the case of using a first pass (in the forward scan) to print a shepherd's check on a white paper and a second pass (in the backward scan) an inverted shepherd's check thereon. The reference numeral 2001 denotes the state of the ink droplet immediately after the printing by the first pass (forward). The part completely filled with black represents cyan ink and the hatched part yellow ink. Since the yellow ink has been injected at the same position as previously occupied by the cyan ink with only a small time lag, the cyan ink is absorbed by the paper in a state of high density with a sign of sparing bleeding and the yellow ink is induced to bleed heavily to the extent of enveloping the lower side and the peripheral part of the cyan ink and eventually assuming a print of low density. Further at this time, these inks are absorbed and spread out so widely as to reach the immediately next picture element, with the result that the entire paper surface will be filled up with the inks as illustrated in Fig. 16. The print made by the second pass (backward) under the condition mentioned above is superposed on the previously absorbed adjacent dots of ink as indicated by 2003. Since the second pass forms a backward scan, the yellow is printed first and the cyan next (2002). When these two inks are left to be absorbed, they eventually assume a state in which they do not appear very conspicuously to the surface as indicated by 2003. In the printed image finally produced, therefore, the density of the first printed cyan is emphasized most strongly and the area of this print forms a green image having a chromatic taste preferentially of cyan. Conversely, in the area of print adjoining the aforementioned area of print which has used the first pass for the backward scan, the cyan and the yellow change their positions and produce a green image having a chromatic taste preferentially of yellow. Fig. 19 depicts the manner in which the two areas of print mentioned above appear. It is clearly noted from this diagram that the lower half part of the head always determines the preferential color in each area and this preferential color is reversed in the forward and backward scan. Since these two areas different in preferential color are alternately present, the phenomenon of uneven coloration still persists in the operation of split printing and impairs the produced image and renders the reciprocating printing virtually infeasible. Further, the drawback caused by the bleeding of ink'in the adjoining picture element is found not only the uneven coloration mentioned above but also in the reciprocating monochromic printing. Now, the trouble caused in this case will be described below. Figs. 20A to 20D illustrate the conditions of ink absorption during a first and a second pass similarly to Figs. 18A to 18C. Likewise in the diagrams, 2101 denotes the condition of ink which has landed on the paper surface by a first pass and 2102 and 2103 both denote cross sections of the paper which are assumed after the printing by a second pass. Here, 2102 represents the state in which the record by the second pass is formed immediately after the record produced by the first pass and 2103 the state in which the record by the second pass is formed after an interval of some length following the formation of the record by the first pass. These two states show a difference in the state of absorption in the paper surface of the ink recorded by the second pass. While the ink dots 2102 are absorbed fairly in the direction of depth of the paper, the ink dots 2103 produced by the second pass are spread out on the surface of the paper. These behaviors of ink dots are discerned on the reverse side of the paper and the ink dots 2103 permeate to the reverse side of the paper to a greater extent than the ink dots 2102. These states of ink dots are manifested as a difference in density of the two inks on the paper surface as shown in Fig. 20C (2104) and Fig. 20D (2105). The time lag arising from the reciprocation of the carriage is ample as compared with the order of the time lag responsible for the difference in density between the two kinds of ink dots. This factor constitutes itself a new drawback attendant on the operation of reciprocating printing. The situation of this drawback will be described below with reference to Fig. 21. As shown in Fig. 21, first the head makes a forward pass from the position of 2201 in the direction of the arrow to effect a record of the first scan width. After the record of one full line has been completed, the paper is fed by one half of the scan width mentioned above and then the head makes a backward scan from the position of 2202 this time in the opposite direction. Again the paper is fed by the same width as mentioned above and the head then makes a forward scan from the position of 2203 to effect a record in the direction of the arrow. The recording intervals of the two passes will be compared below with respect to the parts (1) to (6) in the area of print completed in this case. In the parts (3) and (4), the record by the second pass is commenced immediately after the record by the first pass has been completed and then the paper has been fed by the one half width. In contrast, in the parts (1) and (6), the record by the second pass is commenced after the carriage, subsequent to the record by the first pass, has completed one reciprocating scan. The parts (2) and (5) are recorded with a time lag exactly one half of the duration intervening between the first and the second record. As a result, as shown in Figs. 20A to 20D, the parts (1) and (6) acquire the highest density, followed by the parts (2) and (5), and the parts (3) and (4) absorb inks to a great depth in the paper and acquire a low surface density. Thus, the phenomenon of uneven image density appear in the left-hand area in which the passes (1) and (4) by one half width are repeated in the vertical direction and in the right-hand area in which the passes (3) and (6) are repeated. The uneven density impaired the produced image. The bleeding of the ink of the print by the first pass into the non-printed picture elements is responsible for the fact that the density depends on the recording interval between the first and the second pass. This situation logically explains why the reciprocating printing has not been materialized. The explanation given thus far has assumed the case of applying the reciprocating printing to a monochromic system. The phenomenon in question manifests itself in conjunction with that of uneven coloration even in the case of multicolor recording as already pointed out. In this case, this phenomenon is recognized as prominence of uneven coloration on the left-hand and the right-hand areas or as difference in chromatic taste. Also in the unidirectional recording, the following detrimental factors affect the time lag of recording. The carriage is temporarily suspended when the recording apparatus performs a head recovery scan for the sake of maintaining its own drive in the course of recording or it keeps itself waiting for arrival of record data being transmitted. Then, the suspension of this nature induces irregular occurrence of uneven image density on a still larger order than the inconstancy of time lag described above. To be specific, the carriage enters the phase of suspension as held in the state ensuing from the production of the record by the first pass and, with a certain time lag, the printed area of recording assumes a higher density than the other areas. This phenomenon of uneven image density induced by the factor mentioned above will be hereinafter referred to as "uneven density due to suspension" for the sake of distinction from the uneven density due to time lag described above. In the ink jet recording apparatus which forms an image by driving the recording head in a direction different from the direction in which the nozzles in one head are arranged, an effort to realize the split recording and the reciprocating printing with a view to exalting the quality of image and expediting the printing operation still encounter such image drawbacks as uneven coloration, uneven density due to suspension, and uneven density due to time lag. Further, the reciprocating printing has the possibility of causing positional deviation of ink dots on the paper surface in the forward and the backward printing owing to the accidental rise of the paper from its normal level as pointed out above. When a 100% duty image is subjected to the reciprocating printing by use of the before-mentioned thinning masks incorporating therein ink dot arrays of the patterns of shepherd's check and inverted shepherd's check, the ink dots land on the paper surface in the manner illustrated in Fig. 22. Fig. 22 depicts the case of performing the split recording by the reciprocating printing using the conventional thinning mask of the pattern of a shepherd's check. The diagram shows the ink dots deviating from their normal positions by one quarter of the size of a picture element. The portions in which adjacent ink dots overlap excessively one another and the portions in which wide gaps intervene between adjacent ink dots are made to appear at different positions owing to the use of thinning masks. In the case of Fig. 22, since all the ink dots are printed in a reverse direction relative to the adjacent ink dots, a gap of the size of one ink dot occurs after each ink dot. Thus, the produced image assumes a low density throughout the entire area thereof. The positional deviation of ink dots on the paper surface during the reciprocating printing is caused not only by partial rise or fall of the paper surface illustrated in Figs. 15C and 15D but also by various factors such as, for example, the inconstancy of the speed at which the recording head ejects the ink and the inconstancy of the speed of motion of the carriage. It is not easy to control the timing for discharging the ink during the reciprocating printing because the factors mentioned above are not constant in magnitude relative to the direction of the advance of the carriage. Besides, since the distance from the head to the paper surface in the recording apparatus is dispersed appreciably among the individual apparatuses shipped from the production plant, the control of the landing positions of ink dots in the forward and the backward passes due to the adjustment of the timing for discharging the ink has its own limit. In the conventional recording method which uses such a multi-head as described above, the timing (frequency) for continuous discharge of ink through the individual nozzles is determined by the density of picture elements in the recorded image and the speed of motion of the carriage. If this timing cannot be controlled with amply high accuracy, the ink dots for recording on the surface of paper as the recording medium are incorrectly arrayed relative to the scanning direction of the carriage and the multi-heads, with the result that the recorded image will betray uneven density and inferior quality. As a result, the recording ink dots produced by the head are allowed to form an ideal image array only when the throughput is exalted to the fullest possible extent and the head is driven under conditions such that the limit of frequency of the head and the given density of picture elements may be simultaneously satisfied with high accuracy. Incidentally, in the conventional test print, the method of printing vertical linear patterns perpendicular to the direction of scan as spaced at an interval of not less than several mm is generally adopted for the sake of the test print pattern itself intended to select the optimum conditions and for the purpose of enabling the operator to make his decision as to the selection. Figs. 4A and 4B depict such vertical linear patterns. Specifically, the adversity in question will be briefed below with reference to Figs. 23A, 23B and 24. Figs. 23A and 23B illustrate the manner in which a head 901 fixed on a carriage 706 in motion at a speed S ejects an ink drop at an angle  and a velocity V onto a paper surface placed at a distance P from the head respectively in the forward pass (Fig. 23A) and the backward pass (Fig. 23B). The carriage speed is S in the forward pass and -S conversely in the backward pass and the angle of ejecting is fixed constantly at . Now, let ΔA and ΔB stand for the distance from the position at which the head ejects the ink drop to the position at which the ink drop lands on the paper surface in the forward and the backward pass respectively relative to the direction of scan, then they will be expressed by the following formulas. ΔF = P x (Vsin + S)/Vcos ΔB = P x (Vsin - S)/Vcos Thus, the timing of ink discharge relative to a target picture element in the forward and the backward pass differs in terms of distance as follows. (ΔF - ΔB) = P x 2S/Vcos If this magnitude is invariably constant in all the recording apparatus and recording heads, the positions of ink dots in both directions will be kept duly corrected by driving the head constantly at a timing fit for ideal ink discharge. Actually, however, there exists the possibility that the thickness of a recording paper varies P, the uneven carriage speed varies S, and the inconstancy of craftsmanship among recording heads varies the speed of discharge V. Even one and the same head possibly imparts variation to the speed of ejecting the ink owing to such factors as temperature and direction of scanning or eventually induces gradual variation in this speed owing to the effect of protracted of use. Fig. 24 shows the magnitudes of the distances ΔF and ΔB, the difference (ΔF - ΔB), and the amount of positional deviation of ink dots which are found when the distance P, the carriage speed S, the ejection speed V, and the discharge angle  shown in Figs. 23A and 23B are varied in the forward and the backward pass. The data given in the uppermost row of the table of Fig. 24 represent the case of using the paper distance P = 1.2 mm, the carriage speed S = 4.318 m/sec (equal in forward and backward passes), and the ejection angle  = 10° (equal in forward and backward passes) as conditions and reporting "0" as the magnitude of positional deviation of ink dots in the two directions on the assumption that the head is driven so as to satisfy the magnitudes of ΔF and ΔB and the optimum correction value (ΔF - ΔB) = 84.18 µm. In contrast, the data given in the second and following rows of the table show the fact that the proper correction value (ΔF - ΔB) is varied from one case to another because the magnitudes of various factors are varied little by little. Since the head is driven in these cases with the same timing as that used for the operation yielding the data of the uppermost row, varying amounts of positional deviation of dots inevitably arise. Thus, the magnitudes of deviation indicated in these rows represent differences of optimum correction value (ΔF - ΔB) respectively from the magnitude shown in the uppermost row. In the table of Fig. 24, the individual factor values are varied within the ranges generally accepted for variation of such values. It is remarked from this table that the factor capable of affecting the deviation of dots in the two directions to the greatest extent is the paper distance P. From the table, it is noted that when the paper distance fixed at 1.2 mm is varied by a correction value of only ±0.2 mm, this variation gives rise to a deviation of 42.29 µm (not less than a half picture element at a picture element density of 360 dpi). The recording papers of an ordinary run have a staple thickness of about 100 µm. The variation of thickness of the order just mentioned is easily affected by the inconstancy of the paper distance among the recording apparatuses proper and the inconstancy of craftsmanship among the recording heads. Thus, the corrections must be made in accordance with the conditions of a particular recording apparatus. The variation of the paper distance under discussion which is caused by the inconstancy of the recording apparatus itself can occur while the printing is in process. Ideally, the part of the recording paper engaging in the printing operation should be kept in a flat smooth state by paper retainers disposed one each before and after the site of printing. When the printing duty is high or when the split recording method which completes a print by dividing one and the same recording scan into a plurality of recording scans is adopted, however, the part of the recording paper already used for printing possibly entails shrinkage of fibers therein because of the absorption of the ink. Thus, this particular part is likely to be raised from the normal level. In this case, the paper distance P is apt to vary in the forward and the backward pass in each recording scan. This rise of the paper (hereinafter referred to as "cockling") varies the optimum correction value and consequently gives rise a positional deviation of dots during the reciprocating printing. As described above, the correction value can not be kept constant because of various factors. It has been ascertained that the correction of the positions of ink dots is desirable when the reciprocating printing is carried out or when the record is produced with a plurality of heads. In the case of reciprocating printing or in the case of producing a record by use of a plurality of heads, since such dot positions are evaluated and corrected based on the rectilinearity of vertilines of a specific pattern as described above, the accuracy of determination is inevitably limited and the maintenance of ideal image quality is likewise limited. In short, the conventional method which relies for determination of dot positions on such vertical lines as described above does not easily permit correct determination for the sake of such fine adjustment as involves a microscopic size of several µm or not more than one picture element. Besides, the recent exaction of the manufacture of printed images of improved quality at a great speed has reached a point where a novel method which allows correction of dot positions and, at the same time, permits compensation with such a minute unit as mentioned above serves as an indispensable tool. The conventional printer, on encountering a change in-the material or thickness of the recording medium, requires a varying measure to counter the change and obtains an ideal image condition with difficulty. Thus, the desirability of infallibly and accurately accomplishing ideal recording characteristics without being affected by such factors of the recording medium as material and thickness has been gaining in significance. A system which allows the operator of a printer to evaluate the test print image easily and accurately remains yet to be developed. SUMMARY OF THE INVENTION This invention has been produced in association with the aforementioned drawbacks of the prior art. Thus, it is a concern of this invention to provide an improved ink jet recording method and apparatus. A further concern of this invention is to provide an ink jet recording method and apparatus capable of realizing manufacture of recording images of high quality at a high speed. Another concern of this invention is to provide an ink jet recording method and apparatus capable of rendering inconspicuous the drawbacks of positional deviation of recorded dots possibly posed during the reciprocating printing and imparting exalted quality to the recorded images. Still another concern of this invention is to provide an ink jet recording method and apparatus capable of simultaneously overcoming the defects of image due to the positional deviation of dots during the reciprocating printing and the defects of image such as uneven coloration, uneven recording due to suspension, and uneven recording due to time lag. One aspect of the invention provides an ink jet recording method for recording on a recording medium by reciprocably moving in a main scan direction a recording head having an array of recording elements for ejecting ink extending in a predetermined direction different from the main scan direction, and by causing the recording head to eject ink during forward and backward scans of said recording head in the main scan direction, said method comprising the steps of: recording a first thinned image produced by thinning the image data to be recorded in accordance with a first mask pattern on a predetermined area of said recording medium during a forward scan of said recording head using a first group of said recording elements; feeding said recording medium in a sub-scan direction relative to said recording head by a distance shorter than the recording element array and corresponding to said predetermined area after the first thinned image has been recorded; recording a second thinned image produced by thinning the image data to be recorded in accordance with a second mask pattern on said predetermined area during a backward scan of said recording head using a second group of the recording elements; feeding said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area of said recording medium after the second thinned image has been recorded; recording a third thinned image produced by thinning the image data to be recorded in accordance with a third mask pattern on said predetermined area during a forward scan of the recording head using a third group of the recording elements; feeding said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area after the third thinned image has been recorded; and recording a fourth thinned image produced by thinning the image data to be recorded in accordance with a fourth mask pattern on the predetermined area of said recording medium during a backward scan of said recording head using a fourth group of the recording elements of said recording head, wherein each of the first, second, third and fourth mask patterns defines a pixel array including an arrangement of pixel groups each composed of m, where m ≥ 2, pixels in the main scan direction such that the pixel groups of each mask pattern are not adjacent in the main scan direction and do not overlap one another, each pixel group recorded in accordance with the first mask pattern is adjacent to a pixel group recorded in accordance with the third mask pattern in the main scan direction, and each pixel group recorded in accordance with the second mask pattern is adjacent to a pixel group recorded in accordance with the fourth mask pattern in the main scan direction. Another aspect of the invention provides an ink jet recording apparatus for recording on a recording medium using a recording head having an array of recording elements for each ejecting ink, comprising: main scan means for reciprocably moving the recording head in a main scan direction different from the direction in which the recording element array extends; sub-scan means for feeding said recording medium in a sub-scan direction relative to said recording head; drive means for driving the recording head to cause ink to be ejected from the recording elements in accordance with the image data to be recorded; and control means for controlling the main scan means sub-scan means and drive means for causing a first thinned image produced by thinning the image data to be recorded in accordance with a first mask pattern to be recorded on a predetermined area of said recording medium during a forward scan of the recording head using a first group of said recording elements, for then causing said sub-scan means to feed said recording medium in the sub-scan direction relative to said recording head by a distance shorter than the recording element array and corresponding to said predetermined area, for then causing a second thinned image produced by thinning the image data to be recorded in accordance with a second mask pattern to be recorded on said predetermined area during a backward scan of said recording head using a second group of the recording elements, for then causing said- sub-scan means to feed said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area, for then causing a third thinned image produced by thinning the image data to be recorded in accordance with a third mask pattern to be recorded on said predetermined area during a forward scan of the recording head using a third group of said recording elements, for then causing said sub-scan means to feed said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area and for then causing a fourth thinned image produced by thinning the image data to be recorded in accordance with a fourth mask pattern to be recorded on the predetermined area of said recording medium during a backward scan of the recording head using a fourth group of the recording elements, wherein each of the first, second, third and fourth mask patterns defines a pixel array including an arrangement of pixel groups each composed of m, where m ≥ 2, pixels in the main scan direction such that the pixel groups of each mask pattern are not adjacent in the main scan direction and do not overlap one another and, in use of the apparatus, each pixel group recorded in accordance with the first mask pattern is adjacent to a pixel group recorded in accordance.with the third mask pattern in the main scan direction, and each pixel group recorded in accordance with the second mask pattern is adjacent to a pixel group recorded in accordance with the fourth mask pattern in the main scan direction. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are diagrams for aiding in the description of recording picture elements forming each of the passes used in the four pass printing. Figs. 2A and 2B are diagrams for aiding in the description of recording picture elements forming each of the passes used in the conventional four pass printing. Figs. 3A and 3B are diagrams for aiding in the description of Y, M, C recording picture element arrays forming each of the passes used in the four pass printing in an Embodiment of this invention. Figs. 4A and 4B are diagrams for aiding in the description of recording picture element arrays forming each of the passes in the four pass printing which constitutes itself a precondition of this invention. Figs. 5A and 5B are diagrams for aiding in the description of K recording picture element arrays forming each of the passes used in the four pass printing in Example 2 of this invention. Figs. 6A and 6B are diagrams for aiding in the description of recording picture element arrays used in the four pass printing which constitutes itself a precondition of this invention. Figs. 7A to 7D are diagrams for aiding in the description of C, M, Y unidirectional printing and K bidirectional printing. Fig. 8 is a diagram schematically illustrating the construction of an ink jet recording apparatus to which this invention is applicable. Fig. 9 is a diagram illustrating a multi-head. Fig. 10 is a block diagram illustrating a control part of the ink jet recording apparatus shown in Fig. 8. Figs. 11A to 11C are diagrams illustrating the condition of ideal printing by use of an ink jet printer. Figs. 12A to 12C are diagrams illustrating the condition of a print of uneven density produced by use of an ink jet printer. Figs. 13A to 13C are diagrams illustrating a split printing. Figs. 14A to 14C are diagrams illustrating a split printing. Figs. 15A to 15D are diagrams for aiding in the description of positional deviation of dots deposited on a paper surface in a reciprocating printing. Fig. 16 is a diagram illustrating a thinned array of conventional split records. Figs. 17A to 17D are diagrams illustrating the condition of ink dots in the process of being absorbed on a paper surface. Figs. 18A to 18C are cross sections of a paper surface for aiding in the description of the condition of occurrence of uneven coloration. Fig. 19 is a diagram for aiding in the description of uneven coloration encountered in a conventional split recording. Figs. 20A to 20D are diagrams for aiding in the description of the condition of occurrence of uneven density due to time lag. Fig. 21 is a diagram for aiding in the description of a cause for the uneven density due to the time lag. Fig. 22 is a diagram illustrating positional deviation of dots which have landed on a paper surface in a conventional split recording. Figs. 23A and 23B are diagrams for aiding in the description of deviation of dots encountered in the bidirectional printing by use of an ink jet printer. Fig. 24 is a tabular diagram showing the relation between the cause for dot deviation and the amount of dot deviation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, working embodiments of this invention will be described in detail below with reference to the drawings annexed hereto. Fig. 8 is a perspective view schematically illustrating the construction of an ink jet recording apparatus to which this invention is applicable. In the diagram, 701 stands for each of ink cartridges disposed in array. These ink cartridges 701 comprise ink tanks severally packed with inks of the four colors, black (Bk), cyan (C), magenta (M), and yellow (Y), and multi-heads 702 corresponding to the colors. The appearance of multinozzles 801 disposed on the multi-head as viewed from the direction of Z is illustrated in Fig. 9. The multinozzles 801 are arrayed on the multi-head 702. While the multinozzles 801 are portrayed in this diagram as arrayed parallelly to the Y axis, they may be more or less inclined relative to the XY plane of the diagram. The multinozzles 801, when so inclined, continue their printing actions with their timings successively shifted, whereas the head continues advancing in the direction of advance X. Again with reference to Fig. 8, 703 stands for a paper feed roller which rotates in the direction of the arrow shown in the diagram while keeping a printing paper 707 nipped in cooperation with an auxiliary roller 704 and sends the printing paper 707 in the direction of Y. Paper supply rollers 705 supply a printing paper and, at the same time, discharge the role of nipping the printing paper 707 like the rollers 703 and 704. A carriage 706 supports four ink cartridges and moves them as they continue their printing actions. The carriage 706 is adapted to stand by to await a command at its home position (h) indicated with a dotted line in the diagram while not engaged in the printing work or engaged in the work of restoring the multi-heads. Incidentally, in this working example, the recording heads respectively of the ink jet cartridges are adapted to spout ink drops by causing a change in the state of aggregate of ink by virtue of thermal energy. The four ink jet cartridges mounted on the carriage 706 are so arrayed that, during the forward motion of the carriage, the black ink, cyan ink, magenta ink, and yellow ink spouted through the respective multinozzles 801 may be superposed in the order mentioned. The intermediate colors can be realized by properly superposing the ink dots of the colors C, M, and Y. For example, red is obtained by superposing M and Y, blue C and M, and green C and Y. Generally, black is obtained by superposing the three colors C, M, and Y. The black thus obtained is deficient in development of color. It forms a colored fringe because these colors are not easily superposed with high accuracy. The superposition entails unduly dense injection of inks per unit time. Thus, the-black is exclusively injected independently. Fig. 10 is a block diagram illustrating a control part of the ink jet recording apparatus shown in Fig. 8. In the diagram, 1201 stands for the control part which is constructed as centered around CPU, ROM, RAM, etc. and is operated to control the component parts of the apparatus in accordance with programs stored in the ROM. Denoted by 1202 is a driver for driving a carriage motor 1205 serving the purpose of moving the carriage 706 in the direction X (for main scan) in compliance with a signal from the control part 1201, 1203 a driver for driving a transfer motor 1206 adapted to drive the paper supply roller 705 and the paper feed roller 703 and transfer a recording material in the direction of Y (for sub-scan) in compliance with a signal from the control part 1201, 1204 a driver for driving differently colored multi-heads 1207 to 1210 in compliance with printing data issued from the control part 1201, 1211 an operation display part for displaying inputs from various keys and various data, and 1212 a host device for supplying printing data for the control part 1201. The carriage 706 standing at the home position shown in the drawing and awaiting the start of the printing operation, on receiving an instruction to start the printing operation, moves the direction-X and simultaneously causes n multinozzles 801 on the multi-head 702 to produce a print exclusively within the width D on the paper surface. When the printing of data is terminated at the end part of the paper surface, the carriage returns to the home position and again starts printing data in the direction of X. When the apparatus happens to be in the reciprocating printing mode, the carriage moves in one direction-X and simultaneously continues engaging in the printing operation. Between the time the first round of printing is completed and the time the second round of printing is started, the paper feed roller 703 is rotated in the direction of the arrow to feed a paper in the direction Y in accordance with the width of the recording area. The printing of data on one paper surface is completed by the repetition of the unit cycle of printing and paper feed for each of the passes of the carriage as described above. Now, a concrete example of the recording method to be carried out with such an ink jet recording apparatus as described above will be explained below. (First example) Now, a first example will be described below. Figs. 1A and 1B are diagrams for illustrating the present example in comparison with the diagrams of Figs. 2A and 2B. In Figs. 1A to 2B, four rounds of recording and scanning are carried out on a fixed-area of a recording paper. The recording of an image on this fixed area is attained by sequentially recording a thinned image in each round of recording and scanning with a thinning mask in which picture element arrays are kept in a mutually complementary relation as shown in (a) to (d). In the four rounds (or times) of recording and scanning, the odd number (first and third) rounds of recording and scanning are carried out in the forward pass of the reciprocating motion of the multi-head 702 and the even number (second and fourth) rounds of recording and scanning on the backward pass. The squares of (e) and (f) respectively represent the sum of picture elements recorded in the forward pass and the sum of picture elements recorded in the backward pass. Fig. 1B represents the condition of recording dots which have landed on a paper surface. Fig. 1B like Fig. 2B illustrates the condition in which the reciprocating printing has given rise to positional deviation of dots in one quarter of the picture elements. As regards the condition of aggregate of dots which have landed on the paper surface, the print of Fig. 1B is found to have a smaller number of gaps than the print of Fig. 2B. The difference between the present example (Figs. 1A and 1B) and the example (Figs. 2A and 2B) manifests itself in the sum of recording picture element arrays produced by two rounds of unidirectional printing. In the print of Fig. 2A, the sum of picture element arrays produced in the forward pass and that in the backward pass each form a shepherd's check pattern of one-dot units. In contrast, in the print of Fig. 1A, the sum in the forward pass and the sum in the backward pass each has units each of two picture elements arranged in the direction of scanning without fail. Thus, the phenomenon of thin density is prevented because the picture element arrays resulting from two rounds of unidirectional scanning are formed in mutually adjoining state. The object of this invention resides in rendering inconspicuous to the fullest possible extent the drawback of positional deviation of the dots inevitably caused between the two directions of printing by rise and fall of the paper surface or uneven drive of the paper during the bidirectional printing or by change in the speed of ink ejection. Now, therefore, the question as to what kind of array is generally required for the thinning mask in realizing the object mentioned above will be discussed below. During the bidirectional printing, the deviation occurs unexceptionally on any dots under equal conditions. Thus, the extent to which such gaps as are found in Fig. 2B are decreased determines the object and the effect of this invention. The number of gaps coincides with the number-of points at which the dots deposited on the paper surface in the forward pass and the dots so deposited in the backward pass adjoin each other. In Fig. 2B, the gaps are'formed next all the dots because the prints on the forward pass and those on the backward pass are caused to alternate as spaced at an interval of one dot. In contrast, in Fig. 1B, since the groups of dots printed in the fixed direction are in such a state as have two dots connected to each other in the lateral direction, no gap occurs between these two connected dots and the number of gaps can be reduced at a rate of one to two. Precisely, the number of gaps arising from the deviation of dots during the bidirectional printing is inevitably associated with the number of pairs of laterally connected dots in the sum of the individual picture element arrays formed in the unidirectional printing. As described above, the present embodiment realizes the object of rendering inconspicuous the gaps which occur during the bidirectional printing because the picture element arrays as the sums of picture elements formed in the forward and the backward pass are aggregated as paired off in the direction of the advance of the head for scanning notwithstanding the picture element arrays to be used for each round of recording and scanning comprise one-picture element units which do not adjoin mutually. Besides, the method of the present example is equally effective in the case of monochromic black prints and in the case of prints of a plurality of colors. In the case of a color ink jet recording apparatus, for example, such thinning arrays as are shown in Fig. 1A may be used for black, cyan, magenta, and yellow in common or four picture element arrays of different colors may be circulated in each round of scanning. Further, the use of picture element arrays assuming entirely different complementary relations in each color may be expected to bring about the same effect as mentioned above so long as the picture element arrays for the unidirectional recording and scanning mutually adjoin as already described. (Embodiment) Now, an embodiment of this invention will be cited below. The comparison between Figs. 3A, 3B and Figs. 5A, 5B on the one part and Figs. 4A, 4B and Figs. 6A, 6B on the other part makes the effect of this example stand out. Here, the picture element arrays of Figs. 3A and 3B will be used with respect to cyan, magenta, and yellow and those of Figs. 5A and 5B with respect to black. The difference of this example from Example 1 resides in the fact that the picture element arrays in each round of recording are already formed with 1 x 4 dot groups and the portions of gaps are reduced in advance to one quarter irrespectively of the recording sequence of the individual picture element arrays. Likewise in the present embodiment, the effect of this invention manifests itself between Figs. 3A and 3B on the one part and Figs. 4A and 4B. In the case of Figs. 4A and 4B, the portion in which adjacent dots are recorded by the reverse scanning occurs at a ratio of 1 to 4 picture elements, in the direction of main scan. In contrast, in the case of Figs. 3A and 3B, one aggregate of sums of picture element arrays in the forward scan or sums of picture element arrays in the backward scan forms a chain of 8 picture elements. As a result, the portions allowing the appearance of a gap are reduced proportionately to a ratio of 1 to 8 picture elements. Further, since the present embodiment produces a print with emphasis exclusively on the black ink, it applies a picture element array different from that of other color as shown in Fig. 5A to black. In this case, four rounds of scanning print an equivalent to a total of 200% duty, the individual rounds of scanning are responsible for a print of 50% each. If, in this case, the first two rounds of scanning complete a print of 100% as shown in Fig. 6A, the sum of picture element arrays in the forward printing and the sum of picture element arrays in the backward printing will inevitably equal the picture element arrays in a single round of scanning. If the first two rounds of reciprocating scan produce a print overlapping equal picture elements and the two subsequent rounds of reciprocating scan produce an overlapping print with picture element arrays which are complementary to the first print as illustrated in Fig. 5A, the print in the forward pass and the print in the backward pass will severally complete a 100% image independently of each other and, therefore, absolutely no gap will occur where the dots of the forward and the backward pass mutually deviate. The print of such a duty ratio as exceeds 200% produces the same effect as the present example with respect to the deviation of dots in both the opposite directions even when the picture element arrays in each round of recording and scanning have their component picture elements separated asunder as in the case of the first example, providing the sum of picture elements recorded in the forward pass and the sum of picture elements recorded in the backward pass are each so formed as to complete a 100% print. Figs. 7A to 7D represent the case of producing a unidirectional print with respect to color (C, M, Y) and a bidirectional print with respect to black. In the present example, the multi-head 702 is provided with only eight nozzles and is adapted to produce a reciprocating print in concert with the paper feed which is carried out at a speed of L/2, wherein L stands for the length of the head. This operation of the multi-head 702 is portrayed as continued from the first through the fourth round of recording and scanning. In the first round of recording and scanning, the multi-heads equivalent to four colors are reciprocated by the carriage 706 and, in the meantime, the four out of the total of eight nozzles in the lower half of the multi-head are used to eject inks of relevant colors, K, M, and Y in this particular case, onto the picture elements shown in gray in the first recording area on the recording paper. Then, in the second round of recording and scanning, the multi-head is reciprocated by the carriage 706 and, in the meantime, the nozzles in the lower half of the head are used to eject only the ink of K onto the picture elements recorded in the first round of recording and scanning. In the third round of recording and scanning which follows the paper feed made at the speed of L/2, the multi-head is reciprocated by the carriage 706 and, in the meantime, all the eight nozzles in the head are used to eject the inks of K, M, Y to the picture elements indicated in gray in the first and the second recording areas on the recording paper. Subsequently, in the fourth round of recording and scanning, the multi-head is again reciprocated by the carriage 706 and all the eight nozzles in the head are used to eject the ink of K to the picture elements recorded in the third recording area. The operation described above completes a record in the first recording area. In the present embodiment, the record with black in the forward pass is made in the same picture elements as entirely the same head position as in the backward pass. Even for the emphasis of black effected basically in the unidirectional printing, the present invention operates effectively. It should be remarked that the deviation of dots strictly is not a phenomenon which occurs exclusively during the bidirectional printing. When the printing duty is so high as is recognized today, the warp in the paper surface varies from one to another round of recording and scanning and uneven driving more or less arises among the rounds of recording and scanning. In short, these adverse factors manifest themselves in the form of deviation of dots also during the unidirectional printing. It follows as a consequence that the use in the present example of 1 x 4 picture elements as a basic dot group may well be regarded as effective in countering such deviation of dots as is inevitably encountered even in the unidirectional printing. In the print with color as well as in the print with black, the picture element array based on 1 x 4 picture elements used in the present example is effective in decreasing the number of gaps from the level attainable in the first example. Besides, it can be expected to bring about the following effect. Basically, the split recording method does not fully manifest its effect until the recording picture elements within the unit area are recorded substantially equally among the divisions of scanning. The examples described thus far invariably represent cases of recording a 100% duty image. In any of the cases, therefore, an equal number of picture elements are recorded unexceptionally by four passes. Most image data which are actually transmitted in the form of signals are outcomes of a procedure which comprises binarizing multivalued data representing a certain intensity in accordance with a prescribed method of binarization and shaping the product of binarization in a predetermined pattern. This procedure is effective for the Dither method which finds extensive utility among other methods of the class. Generally by such a method of binarization as the Dither method, picture element arrays corresponding to varying levels of duty are set in such a square matrix as 4 x 4. This matrix is intended to allow realization of an area intensity therein. Thus, the matrix, whenever equal duty values are injected, issue equal picture element layouts without fail. As a result, when equal duty values are injected into all the matrixes such as when uniform patterns are registered, all the 4 x 4 matrixes which are arrayed vertically and laterally on a recorded image record dots of equal picture element arrays and form uniform images. When the 1 x 4 dot group contemplated by the present example is adopted as the basis for picture element arrays in this case, the direction of main scanning of the matrixes mentioned above is tuned to the 1 x 4 dot group and all the pairs of adjacent matrixes are no longer recorded simultaneously in each round of recording and scanning. As a result, the difference in number of dots between successive rounds of recording and scanning does not appear relative to the direction of main scanning. Thus, the recording can be invariably obtained with four kinds of nozzles. The present embodiment realizes the object of rendering inconspicuous the gaps which occur during the bidirectional printing by using 1 x 4 basic picture element arrays in each round of recording and scanning and nevertheless causing the picture element arrays, i.e. the sum of picture elements in the forward pass and that in the backward pass, to be grouped into bound clusters each of eight picture elements in the direction of the advance of the head for scanning as described above. In the present embodiment as in the first embodiment, the four kinds of picture element arrays with relevant colors of black, cyan, magenta, and yellow may be circulated as thinning arrays in each round of scanning. In the present embodiment, as a case of placing emphasis exclusively on black, the picture element arrays of black have been depicted as varied from those of color. The present example, however, is still effective even when prints of all the colors are recorded by the reciprocating printing as illustrated in Figs. 5A and 5B. So far, the fact that the present invention is more effective than the first example has been pointed out. The first example nevertheless proves more effective than the present example such as when the recording paper to be used happens to be an OHP paper which has a poor ability to absorb ink. The present example which is so constructed as to have different ink drops simultaneously recorded as mutually adjoined possibly entails the phenomenon that the adjacent ink dots attract one another and gather into larger ink drops and solidify on the recording medium. In cases of this sort, the first example which allows such ink drops to remain in a mutually independent state may well be rated as more effective than the present embodiment. Explanation has made of the embodiments of the present invention in order to assist in understanding the present invention, it should be noted that the present invention can be applied also for any of combinations of the summary of the invention and the embodiments as mentioned above, and further it can also applied to even thermal transfer type thermal printers. According to the present invention, even though the multi-head in which a plurality of ink jet ports are arrayed is used, in an ink jet recording method in which bidirectional recording scans and relative and successive paper feed are carried out so as to complete recording, the recording of a uniform pattern is completed by carrying out split recording (or respective color multi-heads) by the on-going scan and the in-coming scan for one and the same printing area, the appropriate value for the timing of recording for bidirectional recording (or the appropriate value for the timing of recording for each of the color multi-heads) is determined from the degree of excellency of the uniformities of the above-mentioned patterns (or differences among hues), and is stored in memory, thereby it is possible to carry out the compensation for shifting of dots with a high degree of accuracy which cannot be obtained before, and further it is possible to obtain a high quality image. As mentioned above, the limitation to the conventional determination using the linearity of ruled lines can be overcome, and accordingly, the image can be held to be satisfactory so that fine adjustment less than one pixel, that is, several µm can be made. Further, even though the quality or thickness of recording medium varies, a satisfactory image condition having a satisfactory recording characteristic and a high degree of accuracy can be provided. In particular, in the case of bidirectional printing or in the case of recording with the use of plural color heads, the level of appropriating dot positions can be remarkably enhanced, thereby it is possible to enhance the recording speed for color printing and the quality of image. It is noted that the above-mentioned embodiments have been explained using an ink jet type recording apparatus which utilizes a thermal energy so as to form trajectory liquid drops for printing, as an example among other ink jet recording apparatuses. The typical structure and the principle of the apparatus are preferably those which are disclosed in, for example, U. S. Patent No. 4,723,129 specification or U. S. Patent No. 4,740,796 specification. This system is applicable for either of the so-called on-demand type and continuous type, and in particular, the on-demand type is more effective since membrane boiling is effected at the thermally acting surface of a recording head with the use of thermal energy which is produced from electrothermally energy converting media that are held opposing a sheet or a liquid passage for holding liquid (ink) when at least one drive signal for rapidly increasing a temperature exceeding a nuclei boiling point is applied to the electrothermally energy converting media in accordance with recording data, and as a result, bubbles can be formed in the liquid (ink) which directly corresponds to the drive signal. With the use of the expansion and contraction of the air bubbles, the liquid (ink) can be jetted through jet ports so as to form at least one liquid droplet. It is more preferable if this drive signal has a pulse-like shape since the expansion and contraction of air bubbles can be readily and suitably carried out so that jetting of the liquid (ink) having an excellent responsiveness can be attained. As the pulse-like drive signal, those disclosed in U. S. Patent No. 4,463,359 specification and U. S. Patent No. 4,345,262 specification are more suitable. Further, if the terms disclosed in U. S. Patent No. 4,313,124 specification relating to a temperature rising rate of the above-mentioned thermally acting surface, are used, excellent recording can be performed. As to the structure of the recording head, in addition to the arrangement in which jet ports, liquid passages and electrothermally energy converting media are combined as disclosed in the above-mentioned patent specification (having straight line-like liquid passages or right angle liquid passage), those disclosed in U. S. Patent No. 4,558,333 specification and U. S. Patent No. 4,459,600 specification, which concern such an arrangement that the thermal action is effected in a curved zone, can be used. Further, the arrangement disclosed in Japanese Patent Application Laid-Open No. 59-123670 which concerns common slits that are used as jetting parts of a plurality of electrothermally energy converting media, or the arrangement disclosed in Japanese Patent Application Laid-Open No. 59-138461 which concerns opening holes for absorbing pressure waves having thermal energy, which are opposed to jetting parts can be preferably used. Moreover, a recording head of a replaceable chip type which can be electrically connected with the apparatus body and which can be fed with ink from the apparatus body, or a recording head of a cartridge type with which an ink tank is integrally incorporate can be effectively used in the present invention. Further, the restoring means, an auxiliary assist means or the like may be preferably used for the recording head since the effects of the present invention can be further stabilized. Specifically, these means are such as capping means, cleaning means, pressurizing or sucking means for the recording head, and auxiliary assist means for the electrothermally energy converting media, another heating elements or the combination thereof. Further, the provision of an auxiliary jetting mode with which jetting other than that for recording is effective in order to carry out stable recording. Explanation has been hereinabove made of the use liquid ink. However, ink which is solidified at a temperature equal to or lower than the room temperature, ink which is softened or liquefied at the room temperature, or any other ink which exhibits a liquid phase upon application of a usable recording signal can be also used since the above-mentioned ink jet system is in general adapted to adjust the temperature of ink itself within a range of about 30°C to 70°C so as to control the temperature in order to hold the viscosity of the ink in a stable jetting range. Anyway, ink which prevents a temperature rise caused by thermal energy by positively using the temperature rise as energy for the phase conversion from the solid phase into the liquid phase, or ink which is held in a sold phase in a leaving state in order to prevent the ink from being evaporated, may be used. Anyway there may used ink having such a feature that it is liquefied, at first time, upon application of thermal energy, such as ink which is liquefied in response to the application of recording signal for thermal energy and is jetted as liquid ink, ink which is already solidified at the time of arrival at a recording medium, or the like. In such a case, the ink may be set in such a configuration as disclosed in Japanese Patent Application Laid-Open No. 54-56847 or Japanese Patent Application Laid-Open No. 60-71260, that is, ink is held in a recess part formed of a porous sheet, or in through-holes so as to be opposed to the electrothermally energy converting media. In the present invention, the most effective one of the above-mentioned kinds is such that the above-mentioned membrane boiling is carried out. In addition, the recording apparatus according to the present invention can be used as an image output terminal which is integrally or separatably incorporated with the above-mentioned data processing equipments such as word-processors or computers, or can have a configuration of a copying machine in combination with an image reader, of facsimile machine having signal transmitting and receiving functions. Further, not only the ink jet type using thermal energy, the present invention is also applicable for another ink jet type such as using piezo elements or the like.

Claims: An ink jet recording method for recording on a recording medium by reciprocably moving in a main scan direction a recording head having an array of recording elements for ejecting ink extending in a predetermined direction different from the main scan direction, and by causing the recording head to eject ink during forward and backward scans of said recording head in the main scan direction, said method comprising the steps of: recording a first thinned image produced by thinning the image data to be recorded in accordance with a first mask pattern on a predetermined area of said recording medium during a forward scan of said recording head using a first group of said recording elements; feeding said recording medium in a sub-scan direction relative to said recording head by a distance shorter than the recording element array and corresponding to said predetermined area after the first thinned image has been recorded; recording a second thinned image produced by thinning the image data to be recorded in accordance with a second mask pattern on said predetermined area during a backward scan of said recording head using a second group of the recording elements; feeding said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area of said recording medium after the second thinned image has been recorded; recording a third thinned image produced by thinning the image data to be recorded in accordance with a third mask pattern on said predetermined area during a forward scan of the recording head using a third group of the recording elements; feeding said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area after the third thinned image has been recorded; and recording a fourth thinned image produced by thinning the image data to be recorded in accordance with a fourth mask pattern on the predetermined area of said recording medium during a backward scan of said recording head using a fourth group of the recording elements of said recording head, wherein each of the first, second, third and fourth mask patterns defines a pixel array including an arrangement of pixel groups each composed of m, where m ≥ 2, pixels in the main scan direction such that the pixel groups of each mask pattern are not adjacent in the main scan direction and do not overlap one another, each pixel group recorded in accordance with the first mask pattern is adjacent to a pixel group recorded in accordance with the third mask pattern in the main scan direction, and each pixel group recorded in accordance with the second mask pattern is adjacent to a pixel group recorded in accordance with the fourth mask pattern in the main scan direction. An ink jet recording method according to claim 1, wherein each recording step comprises recording a plurality of droplets in an identical colour in duplication. An ink jet recording method according to claim 1, which comprises using a recording head capable of ejecting inks of different colours including black ink and using the same mask pattern for the first and second mask patterns and the same mask pattern for the third and fourth mask patterns when recording using black ink. An ink jet recording method according to claim 1, 2 or 3, which comprises using as the first, second, third and fourth mask patterns mask patterns for-which m = 4. An ink jet recording method according to claim 1, which comprises using as the first, second, third and fourth mask patterns mask patterns for which a group of four pixels can be recorded out of every sixteen pixels in the main scan direction with successive groups of four pixels being recordable in successive lines of pixels in the direction in which the recording element array extends. An ink jet recording method according to claim 5, which comprises using a recording head capable of ejecting inks of different colours including black ink and using as both the first and the second mask patterns for recording using black ink a combination of the first and fourth mask patterns used for recording using colours of ink other than black and using as both the third and fourth mask patterns for recording using black ink a combination of the second and third mask patterns used for recording with colours of ink other than black. An ink jet recording method according to any preceding claim, which comprises using a recording head having an array of recording elements each of which is capable of imparting thermal energy for ejecting ink and wherein said step of causing the recording head to eject ink comprises applying thermal energy to said ink. An ink jet recording apparatus for recording on a recording medium using a recording head having an array of recording elements for each ejecting ink, comprising: main scan means (706, 1205) for reciprocably moving the recording head in a main scan direction different from the direction in which the recording element array extends; sub-scan means (703) for feeding said recording medium in a sub-scan direction relative to said recording head; drive means (1204) for driving the recording head to cause ink to be ejected from the recording elements in accordance with the image data to be recorded; and control means (1201) for controlling the main scan means (706, 1205) sub-scan means (703) and drive means (1204) for causing a first thinned image produced by thinning the image data to be recorded in accordance with a first mask pattern to be recorded on a predetermined area of said recording medium during a forward scan of the recording head using a first group of said recording elements, for then causing said sub-scan means to feed said recording medium in the sub-scan direction relative to said recording head by a distance shorter than the recording element array and corresponding to said predetermined area, for then causing a second thinned image produced by thinning the image data to be recorded in accordance with a second mask pattern to be recorded on said predetermined area during a backward scan of said recording head using a second group of the recording elements, for then causing said sub-scan means to feed said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area, for then causing a third thinned image produced by thinning the image data to be recorded in accordance with a third mask pattern to be recorded on said predetermined area during a forward scan of the recording head using a third group of said recording elements, for then causing said sub-scan means to feed said recording medium in the sub-scan direction relative to said recording head by a distance corresponding to said predetermined area and for then causing a fourth thinned image produced by thinning the image data to be recorded in accordance with a fourth mask pattern to be recorded on the predetermined area of said recording medium during a backward scan of the recording head using a fourth group of the recording elements, wherein each of the first, second, third and fourth mask patterns defines a pixel array including an arrangement of pixel groups each composed of m, where m ≥ 2, pixels in the main scan direction such that the pixel groups of each mask pattern are not adjacent in the main scan direction and do not overlap one another and, in use of the apparatus, each pixel group recorded in accordance with the first mask pattern is adjacent to a pixel group recorded in accordance with the third mask pattern in the main scan direction, and each pixel group recorded in accordance with the second mask pattern is adjacent to a pixel group recorded in accordance with the fourth mask pattern in the main scan direction. An ink jet recording apparatus according to claim 8, which is operable to record a plurality of droplets in an identical colour in duplication. An ink jet recording apparatus according to claim 8, for recording on a recording medium using a recording head capable of ejecting inks of different colours including black ink and wherein the control means (706, 1205) is operable to cause the first thinned image to be recorded using a first mask pattern which is the same as the second mask pattern and the third thinned image to be recorded using a third mask pattern which is the same as the fourth mask pattern when recording using black ink. An ink jet apparatus according to claim 8, 9 or 10, wherein, in use of the apparatus, the first, second, third and fourth mask patterns are mask patterns for which m = 4. An ink jet recording apparatus according to claim 8, wherein the control means (706, 1205) is operable to cause the first, second, third and fourth mask patterns to be mask patterns for which a group of four pixels can be recorded out of every sixteen pixels in the main scan direction with successive groups of four pixels being recorded in successive lines of pixels in the direction in which the recording element array extends. An ink jet recording apparatus according to claim 12 for recording on a recording medium using a recording head capable of ejecting inks of different colours including black ink, wherein the control means (706, 1205) is operable to cause said first and second mask patterns for recording using black ink to be a combination of the first and fourth mask patterns used for recording using colours of ink other than black and to cause a combination of the second and third mask patterns used for recording with colours of ink other than black to be used as the third and fourth mask patterns for recording using black ink. An ink jet apparatus according to any one of claims 8 to 13, comprising a recording head having an array of recording elements each of which is capable of imparting thermal energy for ejecting ink.

Inventors: HIRABAYASHI HIROMITSU, MATSUBARA MIYUKI, NAGOSHI SHIGEYASU, TORIGOE MAKOTO, HIRABAYASHI, HIROMITSU, MATSUBARA, MIYUKI, NAGOSHI, SHIGEYASU, TORIGOE, MAKOTO, HIRABAYASHI, HIROMITSU, C/O CANON KABUSHIKI KAISHA, MATSUBARA, MIYUKI, C/O CANON KABUSHIKI KAISHA, NAGOSHI, SHIGEYASU, C/O CANON KABUSHIKI KAISHA, TORIGOE, MAKOTO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: G06K15
IPC codes: B41J19, G06K15